The field of the invention is more particularly that of refractory composite materials for use at relatively high temperatures. Such composite materials are constituted by fiber-reinforcement of refractory material densified by a matrix that is likewise refractory and that fills, at least in part, the pores initially present in the fiber reinforcement. The materials from which the fiber reinforcement and the matrix are made are typically selected from carbon and ceramics.
For composite material products containing carbon, it is essential to provide protection against oxidizing in order to avoid the products deteriorating rapidly by the carbon oxidizing whenever the products are used in an oxidizing atmosphere at a temperature exceeding 350.degree. C. Unfortunately refractory composite materials very frequently contain carbon, in particular as a constituent of the fibers forming the fiber reinforcement, or as a constituent of at least a portion of the matrix. A thin layer of carbon may also be formed on the fibers of fiber reinforcement in order to constitute an interphase for providing adequate bonding between the fiber reinforcement and the matrix.
A barrier against ambient oxygen is generally formed by interposing a continuous layer of an oxygen-withstanding ceramic between the carbon contained in the product and the outside surface thereof. This is done either by making at least the outermost portion of the matrix out of such a ceramic, or else by forming an outer coating constituted by said ceramic on the composite material product. The ceramic used is typically a refractory carbide, in particular silicon carbide (SiC). Other carbides are suitable, such as zirconium carbide (ZrC) or hafnium carbide (HfC).
Regardless of whether it constitutes the matrix or merely forms an outer coating on the product, such a layer of refractory carbide is inevitably the seat of microcracking. Microcracks inevitably appear during use of the product due to the mechanical stresses that are applied thereto and to the differences between the thermal expansion coefficients of the constituent materials of the composite. Similar faults may even appear while the product is being made.
Because of the almost inevitable residual porosity of the composite material (in practice the pores initially present in fiber reinforcements are never completely filled by the matrix), the phenomenon of microcracking takes place not only on the surface, but also in the core of the product. Such cracks thus give ambient oxygen access to the underlying carbon.
A known way of solving this problem consists in adding a protective layer that has healing properties for plugging, filling, or sealing the cracks. While the product is in use, varying mechanical and thermal stresses give rise to changes in the shapes of the cracks, in particular the lips of the cracks move together or apart. It is therefore necessary for the healing protective layer to be capable of following such movements without itself cracking. That is why this protective layer is usually made up of elements that constitute a glass or that are suitable for constituting a glass after they have oxidized, with the glass being selected to have viscous behavior at the temperature at which the product is used.
The vitreous healing protective layer nevertheless offers less resistance to abrasion than would a layer of carbide, and while in the viscous state, it also runs the risk of being blown off. Unfortunately, in certain applications, for example parts of aircraft engines or coatings for space aircraft, the surfaces of composite material parts are subjected, in use, to flows of gas at very high speed or they are highly centrifuged, thereby obtaining such a blowing-off effect.
Proposals have therefore been made to provide the healing protective layer with an outer protective coating that withstands abrasion and blowing-off, e.g. an outer coating of a refractory carbide such as SiC. Such an outer coating can be provided, for example, by chemical vapor deposition or infiltration. The composite material product is then protected by a plurality of layers comprising a healing layer that has viscous properties and that lies between two layers of refractory carbide.
An object of the present invention is to provide a method making it possible to protect a carbon-containing composite material against oxidizing, which protection is to be effective over a relatively large temperature range, of about 350.degree. C. to about 1700.degree. C.
Another object of the invention is to provide a method which is easy to implement while nevertheless providing a protective layer presenting both healing properties and high resistance to abrasion and to being blown off.